Enhancement: Provide multi-platform Docker images

The official Docker images are now built for the architectures linux/386,
linux/amd64, linux/arm and linux/arm64.

As an alternative to the Docker Hub, the Docker images are also
available on ghcr.io, the GitHub Container Registry.

https://github.com/restic/restic/issues/2359
https://github.com/restic/restic/issues/4269
https://github.com/restic/restic/pull/4364
